This disclosure relates to fluoropolymers that are used as proton exchange materials in applications such as fuel cells.
Electrochemical devices, such as fuel cells, are commonly used for generating electric current. A single fuel cell typically includes an anode catalyst, a cathode catalyst, and an electrolyte between the anode and cathode catalysts for generating an electric current in a known electrochemical reaction between reactants. The electrolyte can be a proton exchange material, which is also known as or “PEM.”
One common type of polymer exchange material is per-fluorinated sulfonic acid (PFSA), such as NAFION® (E. I. du Pont de Nemours and Company). PFSA polymer consists of a perfluorinated carbon-carbon backbone, to which are attached perfluorinated side chains. Each side chain terminates in a sulfonic acid group that works as a proton exchange site to transfer or conduct protons between the anode and cathode electrodes.
The proton conductivity of PFSA polymers varies in relation to relative humidity (RH) and temperature. The relation between conductivity and level of hydration is based on two different mechanisms of proton transport. One is a vehicular mechanism, where the proton transport is assisted by the water in the membrane, and the other is a hopping mechanism, where the proton hops along the sulfonic acid sites. While the vehicular mechanism is dominant at high relative humidity conditions, the hopping mechanism becomes important at low relative humidity conditions.
PEM fuel cells, especially for automobile applications, are required to be able to operate at high temperature (≧100° C.) and low RH (≦25% RH) conditions, in order to reduce the radiator size, simplify the system construction and improve overall system efficiency. Consequently, PEM materials with high proton conductivity at high temperature and low RH conditions are needed.
PFSA polymer is usually prepared by free radical copolymerization of tetrafluoroethylene (TFE) and per-fluorinated (per-F) vinyl ether monomer (such as perfluoro-2-(2-fluorosulfonylethoxy) propyl vinyl ether, or “PSEPVE”, for NAFION®). One approach to produce a PFSA polymer with improved proton conductivity is to decrease the TFE content in the product polymer. An indicator of conductivity of an electrolyte material is equivalent weight (EW), or grams of polymer required to neutralize 1 mol of base. The most common equivalent weights of commercially available PFSA polymers (such as NAFION®) are between ˜800 and ˜1100 g/mol, which provide a balance between conductivity and mechanical properties. While PFSA polymer with EW in this range is needed, increasing conductivity below a certain EW renders the electrolyte water soluble and not suitable for PEM applications.
Per-F sulfonimide (SI) acids (such as Bis (trifluoromethane) sulfonimide, CF3—SO2—NH—SO2—CF3) show favorable properties, including strong acidity, excellent chemical and electrochemical stability, for PEM fuel cell applications. Linear per-F sulfonimide polymers (PFSI), prepared by copolymerization of TFE and SI-containing per-F vinyl ether monomer, were first reported by DesMarteau, et al. (U.S. Pat. No. 5,463,005). A linear PFSI polymer with EW in the range of 1175-1261 g/mol for PEM application was reported by Creager, et al. (Polymeric materials: science and engineering—WASHINGTON—80, 1999: 600). Per-F vinyl ether monomer that contains two SI groups was also synthesized, and the corresponding linear PFSI polymer with the EW of 1175 g/mol was prepared and demonstrated to have high thermal and chemical stability in PEM fuel cell operating conditions (Zhou, Ph.D. thesis 2002, Clemson University). Reducing TFE content in the PFSI polymers is an efficient way to increase the proton conductivity of the product polymers. Linear PFSI polymer with the EW of 970 g/mol was reported in the literature (Xue, thesis 1996, Clemson University). However, such type of linear PFSI polymer with even lower EW is difficult to synthesize through free-radical copolymerization process and also renders the polymer water soluble below a certain EW threshold.
The preparation of PFSI polymer with calculated EW of ˜1040 by chemical modification of PFSA polymer resin (in —SO2—F form) was reported in a Japanese patent (Publication No: 2002212234). Furthermore, a more efficient chemical modification process was reported by Hamrock et al. (Publication No. WO 2011/129967). In this process, a linear PFSA polymer resin (in —SO2—F form) was treated with ammonia in acetonitrile (ACN) to convert the —SO2—F groups to sulfonamide (—SO2—NH2) groups, which then reacted with a per-F disulfonyl difluoride compound (such as F—SO2—(CF2)3—SO2—F) to convert to —SO2—NH—SO2—(CF2)3—SO3H in the final product. By starting with PFSA (in —SO2—F form) with EW of 800 g/mol, water-insoluble polymer electrolyte with EW as low as ˜625 g/mol was reported. However, polymer electrolyte with even lower EW (<625 g/mol) resulted in a water soluble polymer and hence is not suitable for PEM applications.